Catalytic dechlorination of 1,2-DCA in nano Cu0-borohydride system: effects of Cu0/Cun+ ratio, surface poisoning, and regeneration of Cu0 sites

Aqueous-phase catalyzed reduction of organic contaminants via zerovalent copper nanoparticles (nCu0), coupled with borohydride (hydrogen donor), has shown promising results. So far, the research on nCu0 as a remedial treatment has focused mainly on contaminant removal efficiencies and degradation mechanisms. Our study has examined the effects of Cu0/Cun+ ratio, surface poisoning (presence of chloride, sulfides, humic acid (HA)), and regeneration of Cu0 sites on catalytic dechlorination of aqueous-phase 1,2-dichloroethane (1,2-DCA) via nCu0-borohydride. Scanning electron microscopy confirmed the nano size and quasi-spherical shape of nCu0 particles. X-ray diffraction confirmed the presence of Cu0 and Cu2O and x-ray photoelectron spectroscopy also provided the Cu0/Cun+ ratios. Reactivity experiments showed that nCu0 was incapable of utilizing H2 from borohydride left over during nCu0 synthesis and, hence, additional borohydride was essential for 1,2-DCA dechlorination. Washing the nCu0 particles improved their Cu0/Cun+ ratio (1.27) and 92% 1,2-DCA was removed in 7 h with kobs = 0.345 h−1 as compared to only 44% by unwashed nCu0 (0.158 h−1) with Cu0/Cun+ ratio of 0.59, in the presence of borohydride. The presence of chloride (1000–2000 mg L−1), sulfides (0.4–4 mg L−1), and HA (10–30 mg L−1) suppressed 1,2-DCA dechlorination; which was improved by additional borohydride probably via regeneration of Cu0 sites. Coating the particles decreased their catalytic dechlorination efficiency. 85–90% of the removed 1,2-DCA was recovered as chloride. Chloroethane and ethane were main dechlorination products indicating hydrogenolysis as the major pathway. Our results imply that synthesis parameters and groundwater solutes control nCu0 catalytic activity by altering its physico-chemical properties. Thus, these factors should be considered to develop an efficient remedial design for practical applications of nCu0-borohydride.

(2) Bare, washed nCu 0 (B-nCu 0 W ) These particles were prepared by washing the freshly synthesized B-nCu 0 particles thrice with DD water.
(3) CMC-coated, washed nCu 0 (C-nCu 0 W ) Freshly synthesized B-nCu 0 W particles were mixed with the CMC solution (0.5% weight/volume) by continuously stirring for 30 min. The washing step was performed before coating the particles with CMC.
(4) CMC-coated nCu 0 (C-nCu 0 ) These particles were synthesized by thoroughly mixing the CMC solution with the CuSO 4 solution to form a Cu 2+ -CMC complex. Then NaBH 4 solution was added dropwise and the suspension was stirred continuously for additional 15 min.
For the type 4 and 5 particles, the supernatant solution from synthesis was retained with the particles to utilize the residual borohydride as a H 2 source for 1,2-DCA dechlorination (Exp. 15-17, Table 1).
Particle characterization. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/ EDX). Hitachi S-4500 field emission SEM equipped with a Quartz PCI XOne SSD X-ray analyzer (Hitachi Ltd., Tokyo, Japan) was used to determine the particle size and surface morphology of B-nCu 0 and B-nCu 0 W particles. Samples were prepared by sprinkling solid samples onto adhesive carbon tape supported on a metallic disk and examined at 10 kV accelerating voltage. EDX was used in conjunction with SEM to determine the elemental composition of the particles by randomly selecting areas on the solid surface.  15 and 16. b Fresh borohydride solution (to get 25 mM) was re-injected into the reactor bottles at t = 24.5 h. Total degradation after 31 h is also shown here. c Fresh borohydride solution (to get 25 mM) was injected into the reactor bottles at t = 16 h. Total degradation after 24 h is also shown here. d Pd = 0.5% w/w of Cu. e Fresh borohydride solution (to get 25 mM) was re-injected into the reactor bottles at t = 13 h. Total degradation after 16 h is also shown here. f Loading dose of C-nCu 0 W was 0.1 g L −1 .  www.nature.com/scientificreports/ X-Ray diffraction (XRD). A Rigaku RPT 300 RC diffractometer (Rigaku, Tokyo, Japan), using Cu Kα radiation, step size 0.02°, and 2θ range 10-90°, was used to determine the product phase composition of B-nCu 0 and B-nCu 0 W particles before and after the dechlorination reaction. The identification of phases was carried out by comparing the experimental data with the JCPDS (Joint Committee on Powder Diffraction Standards) database and the published literature 10,17,54 .
X-ray Photoelectron spectroscopy (XPS). XPS was performed on unreacted B-nCu 0 and B-nCu 0 W particles to analyze their chemical composition and the oxidation state of the elements present. Samples were prepared and introduced into the spectrometer via an anaerobic glove box. A Kratos Axis Ultra spectrometer (Kratos Analytical, Manchester, UK) using a monochromatic Al Kα source (15 mA, 14 kV) was used to carry out the XPS analysis. The instrument work function was calibrated to give an Au 4f7/2 metallic gold binding energy (BE) of 83.96 eV and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for metallic Cu 2p3/2. The Kratos charge neutralizer system was used for all analyses. Spectra were charge corrected to the main line of the carbon 1S spectrum (adventitious carbon) set to 284.8 eV. The XPS survey scans were collected using an analysis area of 300 × 700 microns, a pass energy of 160 eV, and BE range of 1100-0 eV. High resolution spectra were obtained using an analysis area of 300 × 700 μm and a 10 or 20 eV pass energy (20 eV was used for Cu LMM Auger spectral results). Spectra were analyzed using CasaXPS software (version 2.3.14).
Transmission electron microscopy (TEM). A Philips CM10 and a FEI Titan 80-300 Cryo-in-situ TEM (Philips Export B.V. Eindhove, Netherlands) was used to determine surface morphology and size of the C-nCu 0 particles before and after the dechlorination reaction. Samples were prepared in an anaerobic glove box by adding a drop of freshly synthesized diluted nanoparticle suspension on 400 mesh Formvar/Carbon film grids and then the grids were left to dry for some time. A Hamamatsu CCD based camera system software (Advanced Microscopy Techniques, version AMTV542) was used to determine the diameters of the nanoparticles from the obtained micrographs. Table 1  Analytical methods. 1,2-DCA concentrations were measured using an Agilent 7890 Gas Chromatograph (GC, Agilent Technologies, Canada) equipped with a DB-624 capillary column (75 m × 0.45 mm × 2.55 µm) and an Electron Capture Detector (ECD). 250 μL aliquot was collected from each reactor bottle at a selected sampling time and mixed with 1 mL n-Hexane in a 2-mL GC vial. The GC vials were vortex-mixed and allowed to equilibrate for 2 h before extraction. One μL of the extract was injected into the GC using an autosampler. 250 μL samples, directly withdrawn from the headspace of the reactor bottles, were manually injected to analyze concentrations of chloroethane (CE), ethane, ethene, and other hydrocarbons with an Agilent 7890 GC (Agilent Technologies, Canada) equipped with a GS-Gas Pro Column (3.0 m × 320 μm) and Flame Ionization Detector (FID).

1,2-DCA dechlorination experiments.
Chloride analysis was performed using high-performance liquid chromatography (HPLC) equipped with a conductivity detector (Model 432, Waters, Milford, MA), an IC PakTM anion column (4.6 m × 50 mm), and 12% acetonitrile eluent. Samples for chloride analysis were collected at the end of the dechlorination experiments. A Yellow Spring Instrument (YSI Incorporated, USA) probe Model number 85 was used to measure dissolved oxygen in deionized water.

Results and discussion
Characterization of bare nCu 0 . Physico-chemical properties (e.g., shape, size, chemical composition, metal oxidation state) of metals can strongly affect their catalytic characteristics and, consequently, their contaminant removal efficiencies 11,18,41 . Particles were characterized using SEM/EDX, XRD, and XPS to provide an improved understanding of these properties. www.nature.com/scientificreports/ Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). SEM images (Figs. 1A,B) of B-nCu 0 W show agglomeration of particles in the absence of a coating, also reported by Huang et al. 16,17 . Nanoparticles were quasi spherical, with uniform shape and size (diameter 20-40 nm), and assembled in chains forming tightly packed aggregates. No major change in shape and size of reacted particles (Fig. 1B) indicates that borohydride addition at t = 0 h inhibited any significant oxidation of B-nCu 0 W during dechlorination. Past research also found that the morphological integrity of metal nanoparticles was retained in the presence of borohydride even after contaminant degradation [20][21][22] . EDX spectra showed Cu as the major species, accounting for 98% (weight-percent) for both reacted and unreacted B-nCu 0 W ( Fig. 1D-E; Supplementary Information (SI): Table S1). Furthermore, no sulfur and lesser oxygen in B-nCu 0 W clearly indicates that washing step helped in www.nature.com/scientificreports/ removing surface impurities. Figure 1C shows that unreacted B-nCu 0 particles were quite similar to B-nCu 0 W in shape and size. However, small amount of sulfur (0.52%) was found as impurity on B-nCu 0 ( Fig. 1F; SI: Table S1) which came from precursor CuSO 4 .

X-ray diffraction (XRD).
Diffractograms showed characteristic peaks of Cu 0 which matched with those of standard JCPDS card 04-0836 and published literature (SI : Table S2), thus, confirming the formation of metallic Cu (Cu 0 ) for both B-nCu 0 W and B-nCu 0 ( Fig. 2A,B). Three peaks (2θ) at 43.4º, 50.4º, and 74.1º showed the (111), (200), and (220) planes of Cu 0 , indicating the face-centred cubic (fcc) of the nanoparticles 10,17,54 . Cuprous oxide (Cu 2 O) was also present in both samples, with the peaks consistent with JCPDS card 05-0667 and literature values (SI : Table S2). Peaks at 36.5º, 42.3º, 61.4º, and 73.3º showed the (111), (200), (220), and (311) planes corresponding to Cu 2 O 10,17,54 . Peak intensities for B-nCu 0 W and B-nCu 0 particles were different, indicating different ratios of Cu species in the two types of particles. XRD of reacted B-nCu 0 W showed some decrease in intensity of Cu 2 O peaks but an increase in the intensity of Cu 0 peaks (Fig. 2C), also reported by Huang et al. 17 . This further confirms that freshly injected borohydride resulted in preservation and/or regeneration of Cu 0 during dechlorination. Past research also reported prevention of nanometals oxidation while treating p-nitrophenol in the presence of NaBH 4 20,22 . X-ray photoelectron spectroscopy (XPS). XPS determined the distribution of different Cu species (Cu 0 , Cu + , and Cu 2+ ) and the elemental composition on surfaces of B-nCu 0 and B-nCu 0 W ( Fig. 3; SI: Table S3, Figures S1-S2). In high resolution Cu 2p spectra for B-nCu 0 and B-nCu 0 W ( Fig. 3A1 and B1), Cu 2p 3/2 peak located at ~ 932.5 eV for Cu 0 and Cu + could not be resolved, thus, making it impossible to distinguish between these two species. Past research also reported about the almost identical BE values for Cu 0 and Cu + species in the Cu 2p spectra [55][56][57][58] .
Thus, x-ray induced auger electron spectroscopy (XAES) for Cu LMM was adopted to confirm the presence of different valence states of Cu in our samples. The Auger binding energy peak was broad and asymmetric in the range of 563 to 578 eV ( Fig. 3A2 and B2), implying the coexistence of Cu 0 and Cu + in both the samples. Deconvolution of the asymmetric peak resulted in two major peaks at ~ 567.9 and ~ 569.9 eV, assigned to Cu 0 and Cu + respectively 55,56,58,59 . Other minor peaks at ~ 568.8 and 573.6 were assigned to Cu + and the peaks at ~ 565.3, 567, 568.5, 570.4, and 572.4 eV to Cu 055,59 . This confirms the presence of Cu 0 as well as Cu 2 O in both samples, also evidenced by XRD (Fig. 2). The XPS core level spectra ( Fig. 3A1 and B1) also displayed a shake-up satellite at ~ 946.1 eV, indicating the presence of Cu 2+ species (CuO and/or Cu(OH) 2 ) 55,56 . The Cu 2p 3/2 core level signal as well as an auger peak for CuO were not developed, suggesting that Cu 2+ species was present as Cu(OH) 2 . This is further confirmed by presence of a major signal at ~ 570.2 eV for the Cu LMM auger transition in Fig. 3A2 and B2 56 . Alkaline pH of nCu 0 suspensions would have favored Cu(OH) 2 formation. The Cu 0 /Cu n+ area ratios (Cu n+ = Cu + + Cu 2+ ), derived from fitting the Cu LMM peaks, were found to be 1.27 and 0.59 for B-nCu 0 W and B-nCu 0 , respectively. The higher Cu 0 /Cu n+ ratio for B-nCu 0 W strongly suggests that washing the particles with DD water removed oxidized species from nCu 0 surface and, consequently, increased the proportion of Cu 0 which provides sites for H 2 activation and is responsible for catalytic activity. www.nature.com/scientificreports/ lar trend was observed by Woo et al. 31 for nitrate removal while testing water-washed versus solvent-washed nZVI. These trends were attributed to the changes in the physico-chemical properties of the nZVI, which were caused by the washing step. El-Sharnouby et al. 11 reported a significantly slower and incomplete dechlorination of 1,2-DCA by the washed nPd 0 than the unwashed nPd 0 , in the presence of borohydride. These studies clearly indicate that the washing of nanometal particles with water did not favor contaminant removal and, thus, this step may need to be excluded or modified. To our knowledge, all the past dechlorination studies with the nCu 0 included the water-washing step during synthesis but none of them investigated its effects on the properties and dechlorination efficiency of nCu 010,12,14-17 .
Washing nCu 0 particles with DD water significantly enhanced 1,2-DCA dechlorination ( Fig. 4A; Table 1), with ~ 92% removal by B-nCu 0 W -borohydride (Experiment 2) in 7 h as compared to only 44% by B-nCu 0 -borohydride (Experiment 1). Interestingly, these results were in contrast with the findings from our previous study 11 , in which nPd 0 was synthesized by the same method and borohydride was used as the H 2 source.
Both the zerovalent (M 0 ) and the electron-deficient (M n+ ) sites are essential for catalytic dechlorination reactions as M 0 sites facilitate the dissociation of H 2 into the robust reductant (H * ) and M n+ sites dissociatively adsorb the cVOC molecules 18 . An optimum ratio of these two is required for effective catalytic reactions, depending on the nature of the catalyst and the reactant. Pd 0 and rhodium (Rh 0 ), with strong H 2 adsorption/activation properties 60 , need lesser number of M 0 sites for better dechlorination, as reported for 4-chlorophenol 61,62 . Dechlorination decreased with increase in the M 0 /M n+ ratio for Pd 0 and Rh 011,61,62 , probably due to the extensive surface coverage by H 2 gas and limited surface sites for the contaminant adsorption. However, metals like copper and zinc with weaker H 2 adsorption/activation properties 60 may need more M 0 sites. In our study, the washing step had a strong influence on the distribution of Cu species (Cu 0 , Cu + , Cu 2+ ) where B-nCu 0 W had 1.5 times more metallic Cu (Cu 0 ) in the surface layer than the B-nCu 0 as revealed by the XPS analysis (Fig. 3). The higher number of Cu 0 sites in B-nCu 0 W seemed to have favored the availability of H * required for the dechlorination of 1,2-DCA (Eqs. 1-2).
The intensity of Cu 0 peaks was also comparatively stronger in XRD of B-nCu 0 W (Fig. 2). Furthermore, EDX analysis ( Fig. 1F; SI: Table S1) showed lower oxygen content in the B-nCu 0 W . Also, sulfur (0.5%) was present as an impurity on the B-nCu 0 surface. No sulfur found in B-nCu 0 W (Fig. 1D) suggests that washing might have helped in removing sulfur compounds from the surface. Past research reported that sulfur poisoning of Cu surface can negatively influence its catalytic activity 35,36 .
Thus, the higher catalytic dechlorination efficiency of B-nCu 0 W than B-nCu 0 can be attributed to the washing step, which changed the chemical composition (particularly the Cu 0 /Cu n+ ratio) of nCu 0 surface by effectively removing the impurities (e.g., sulfur, oxygen, boron) from the particle surface, which otherwise would have blocked the reactive sites and/or favored nCu 0 oxidation.   Figures S3-S4), with very similar dechlorination rates and extent (Fig. 4A-B; Table 1). Najafi and Azizian 24 also observed this during reduction of 4-nitrophenol on Cu/Cu 2 O nanoparticle surface. However, past research also reported a decrease in the rate constant with an increase in contaminant concentration, attributing it to reactive site saturation 63 . In our study, no significant change in k obs at higher 1,2-DCA concentrations indicates that site saturation was not yet reached for B-nCu 0 W at t = 7 h. However, dechlorination for all the initial 1,2-DCA concentrations almost halted at t = 24 h which could be due to deactivation of reactive sites after longer exposure.
Chloride Mass Balance To confirm 1,2-DCA dechlorination, Cl − concentrations were measured at t = 48 h (Fig. 4C). 37-86% of total chloride (based on two moles of chloride per mole of 1,2-DCA) was recovered which accounted for 85-90% of the degraded 1,2-DCA. The 7-13% unaccounted chloride could be present as chloroethane, an intermediate of 1,2-DCA dechlorination. Shee et al. 14 also reported 10-15% conversion of DCM and 1,1-DCA to monochloromethane and monochloroethane, respectively, during dehalogenation by nCu 0 -borohydride. Thus, 7-13% of the recovered chloride would have come from incomplete 1,2-DCA dechlorination and remaining from its complete dechlorination. While treating DCM with nCu 0 -borohyride, Huang et al. 17 recovered 75% chloride which was attributed to both the complete dechlorination to hydrocarbons and the incomplete dechlorination to chloromethane. www.nature.com/scientificreports/ had retained 80% catalytic activity in the presence of 2% sulfur but Cu/Al 2 O 3 catalyst was completely deactivated with only 0.2% sulfur 35,64 . Chen et al. 65 reported higher HA adsorption on nZVI than micro-ZVI surface which resulted in lesser H * formation on the nZVI and, thus, lesser contaminant degradation. Though extensive work has been published on the effects of groundwater constituents on various copper catalysts (mostly supported catalysts), no study has investigated their effects on the catalytic efficiency of nCu 0 synthesized by borohydride reduction method. Thus, the effects of groundwater constituents (Cl − , S 2− , HA; Experiments 5-13) on 1,2-DCA dechlorination, catalyzed via B-nCu 0 W -borohydride, were evaluated. This also helped in exploring whether the additional borohydride, added as an H 2 source, had any positive impact on the surface poisoning of B-nCu 0 W . Though XPS or XRD analyses were not conducted for these experiments to confirm the surface poisoning, the effects of groundwater constituents on the catalytic efficiency of B-nCu 0 W have been discussed comprehensively based on past literature.
Effect of Chlorides At 1000 mg L −1 Cl − , 1,2-DCA removal efficiency declined to 77.3% and k obs decreased to 0.207 h −1 (Fig. 5A). Increasing Cl − concentrations resulted in a further decrease in both the 1,2-DCA removal as well as the k obs . This can be attributed to surface poisoning of B-nCu 0 W by Cl − , which can be aggressive to copper even in trace amounts. Chloride poisoning of Cu catalysts can occur by several parallel mechanisms including physical blocking and modification of catalytic sites by Cu-Cl complex formation 35,66 . Previous XPS studies showed that oxidized copper species were responsible for chemical reaction with chlorides and physical blocking (molecular adsorption) took place in absence of surface oxygen 57,67 . In our study, the presence of oxidized copper species (Cu 2 O and Cu(OH) 2 ) on B-nCu 0 W surface (Figs. 2 and 3), suggests that chemical interaction with Cl − could be the major surface poisoning mechanism. Cl − forms an unstable CuCl film (Eq. 7), by interacting with Cu 2 O layer on the copper surface, accompanied by other cuprous chloride complexes such as CuCl 2 − , CuCl 3 2− , and CuCl 4 3− (Eq. 8) at higher Cl − concentrations 57,68 . The amorphous Cu(OH) 2 layer can also be easily attacked by Cl − to form soluble CuCl 2 or CuCl 2 ·3Cu(OH) 2 known as atacamite (Eqs. 9-10). Similar to chloride poisoning mechanisms, sulfide poisoning of Cu surface can also occur due to strong chemical bonding of sulfur with oxidized copper species or by physical blocking due to adsorbed sulfur 67,69 . In our study, the oxidized copper species present on B-nCu 0 W surface would have strongly favored the chemical interaction with sulfide. Previous X-ray absorption spectroscopy (XAS) studies reported significant conversion of surface Cu 2 O and Cu(OH) 2 to Cu x S y (Eqs. [12][13][14], after exposing copper metal to an aqueous sulfide solution at alkaline pH 70,71 . Hollmark et al. 72 also proposed the possibility of a copper oxysulfide compound (Cu-O-S) formation as well as direct interaction of sulfide with the underlying Cu 0 . Prašnikara and Likozar 73 reported that Cu(111) plane provides sites for H 2 activation and the sulfide adsorption on these planes could reduce activation and, consequently, affect catalytic efficiency of Cu catalysts.
Effect of Humic Acid Humic acids can prevent particle aggregation, resulting in increased available surface area and improved subsurface transport 37 . However, humic acids can decrease the catalytic activity of nanoparticles as observed in our study. Both the 1,2-DCA removal as well as the k obs values decreased with increase in HA concentrations ( Fig. 5C; Table 1). Various possible mechanisms can be responsible for this decrease in catalytic dechlorination. Humic substances are reported to rapidly dissolve metal nanoparticles, resulting in their oxidation 34,37 . Pradhan et al. 37 also reported multilayer surface adsorption of HA on nCu 0 surface. The surface adsorbed HA would block reactive sites on metal surface, thus, forming an electron transfer barrier for contaminants 34,74 . Increased HA concentrations would also increase cVOCs partitioning into bulk aqueous phase, thus, limiting their concentration on metal surface 75 . Humic acid functional groups would also directly compete with contaminants for reactive sites 38 . Humic acids could also act as competitive H 2 and electron acceptors 74 .
Regeneration of B-nCu 0 W after surface poisoning. Different agents (physical or chemical) are used for regenerating the different types of copper catalysts. In the case of supported copper catalysts, the support itself can sometimes avoid surface poisoning or can act as a regenerating agent. The Cu/ZnO type catalysts are expected to not lose much activity due to sulfur poisoning as sulfur can be taken up as zinc sulfide 35,64 . Magnesium served as a chlorine sink for the Cu x Mg 1-x Al 2 O 4 76 . Various oxidants like hypochlorite and permanganate are used to regenerate the sulfur-poisoned catalysts as surface bound sulfur (mostly sulfides) can be oxidized to sulfate. However, these oxidants cannot be used for all types of catalysts as they also oxidize the M 0 sites. NaOCl was reported to cause Cu dissolution from a Cu-Pd/Al 2 O 3 catalyst 77 . Chlorine-poisoned catalysts can be successfully regenerated by heated H 2 or H 2 /N 2 18,78 , without causing any oxidation. However, using the heated gas can be tedious and unsafe. Past research has briefly discussed about the regeneration of sulfide-poisoned copper (supported) catalysts by NaBH 4 78,79 . As NaBH 4 is a strong reducing agent, it would not remove the sulfides (reduced species) by transforming them to an oxidized soluble species such as sulfate. We have further attempted to investigate the NaBH 4 regeneration of the unsupported nCu 0 catalyst (poisoned by chlorine, sulfur, and HA), in terms of dechlorination efficiency as well as by examining the properties of regenerated catalyst by SEM, EDX, and XRD analyses. In experiments 7, 10, and 13, borohydride (25 mM) was re-injected at t = 24.5 h to regenerate Cu 0 sites of B-nCu 0 W . In presence of 2000 mg L −1 chloride (Experiment 7), borohydride re-injection continued the 1,2-DCA dechlorination with a final removal of ~ 95% in 31 h (Fig. 5D). The unstable CuCl film, formed on metal surface in Cl − presence, can be transformed to metallic Cu in the presence of H 2 80 . A reaction between CuCl and water forms Cu 2 O on metal surface and, then, this newly formed oxide layer is reduced to metallic copper by H 2 (g) (Eqs. [15][16]. XRD of reacted B-nCu 0 W shows significant Cu 0 peaks (SI: Figure S6A) indicating that H 2 from additional borohydride (at t = 24.5 h) resulted in the regeneration of Cu 0 sites, according to Eq. 16. SEM and EDX data (SI: Figures S7A, S7D, Table S1) also show that shape, size, and chemical composition of the reacted B-nCu 0 W did not change much. www.nature.com/scientificreports/ In presence of 4 mg L −1 sulfide (Experiment 10), 1,2-DCA dechlorination completely halted at t = 5 h after removing ~ 42% 1,2-DCA (Fig. 5D). However, borohydride re-injection resulted in ~ 81% 1,2-DCA removal in 29 h. It is worth mentioning that the reaction again slowed down at t = 31 h (82.1% removal), indicating the re-poisoning of Cu surface with the sulfides in this closed system. XRD of reacted B-nCu 0 W did not show any Cu x S y peaks (SI: Figure S6B), suggesting that sulfide poisoning might have not reached the bulk of the material. Kristiansen et al. 71 also did not observe any significant change in XRD of copper metal after sulfidation, however, XAS confirmed the formation of Cu 2 S and CuS. SEM image (SI: Figure S7B) shows that most particles still retained their shape and size after dechlorination. Presence of sulfur (0.22%) (SI: Figure S7E, Table S1) further confirmed the poisoning of Cu surface with sulfide.
In presence of 30 mg L −1 humic acid (Experiment 13), 1,2-DCA dechlorination stopped at t = 7 h (Fig. 5D). However, borohydride re-injection resulted in ~ 93% 1,2-DCA removal in 31 h. This can be attributed to regeneration of Cu 0 sites, as the oxidized Cu (Cu 2+ , Cu + ) released due to dissolution by HA would be reductively transformed to Cu 0 by H 2 . XRD of reacted B-nCu 0 W further supports this by showing a higher intensity of Cu 0 peaks than Cu 2 O peaks (SI: Figure S6C). SEM and EDX data (SI: Figures S7C, S7F, Table S1) confirmed the preservation of particle shape, size, and chemical composition even after dechlorination.
CMC-coated copper nanoparticles (C-nCu 0 ). TEM image of unreacted C-nCu 0 shows individual, spherical nano-sized particles (diameter = 9.07 ± 2.36 nm) assembled in chains, forming some loose aggregates (SI: Figure S8). After dechlorination, most particles agglomerated into larger chunks with no specific shape and size (SI: Figure S8B), indicating oxidation or phase transformation of C-nCu 0 . Inset figure shows that some individual spherical nanoparticles, of increased size, were still present after dechlorination. The physico-chemical properties for C-nCu 0 can be found in the SI Text S1.
1,2-DCA dechlorination. C-nCu 0 was tested for catalyzing 1,2-DCA dechlorination by utilizing H 2 generated from the NaBH 4 left unused during particle synthesis (Experiment 15). This treatment failed to dechlorinate 1,2-DCA, resulting in < 5% removal in 16 h (Fig. 6). It can be attributed to milder hydrogenation properties of Cu 17,35 as compared to Pd, as 1,2-DCA was found to be completely removed by nPd 0 under similar conditions 11 . Lopez-Ruiz et al. 60 reported that hydrogen binds more strongly to Pd (-0.74 eV) than to Cu (-0.37 eV). nCu 0 might have not been able to adsorb sufficient H 2 , formed from hydrolysis of unused NaBH 4 during synthesis, and H 2 escaped from the open system used for C-nCu 0 synthesis in glove box. Addition of fresh borohydride solution at t = 16 h resulted in 32% 1,2-DCA removal at t = 24 h but the reaction ceased thereafter. Interestingly, even when C-nCu 0 was doped with Pd (0.5 w/w%; Experiment 16) to help retain H 2 gas formed during synthesis (as observed by El-Sharnouby et al. 11 ), no appreciable dechlorination occurred (Fig. 6). Similar to C-nCu 0 , borohydride injection at t = 16 h resulted in 37% 1,2-DCA removal but dechlorination halted again. Possible reasons for lower 1,2-DCA removal, even after borohydride addition, could be: (1) lower Cu 0 /Cu n+ ratio, (2) presence of surface impurities (like S) due to exclusion of washing step during synthesis (Discussed in the sections above), and (3) interference by polymer coating.
To study the effects of washing and coating, a washing step was included in synthesis of C-nCu 0 W (Experiment 18). With borohydride addition at t = 0 h, 73% 1,2-DCA was removed in 5 h but the reaction halted thereafter (Fig. 6). The rate and extent of dechlorination were better than the other CMC-coated treatments (Experiments [15][16][17]. This indicates that washing step improved catalytic dechlorination efficiency by increasing Cu 0 /Cu n+ ratio and removing surface impurities. Under similar experimental conditions, dechlorination rate and extent for C-nCu 0 W (Experiment 18) were lower than that for B-nCu 0 W (Experiment 2), indicating a negative impact of CMC coating on 1,2-DCA dechlorination.
Coating the nanometals prevents their agglomeration and generally results in greater reactivity and better subsurface mobility 4,41,46 . However, polymer coatings can also decrease catalyst activity by blocking active surface sites, especially the post-synthesis coatings 11,47 . Moreover, diffusion of aqueous-phase contaminants to metal surface can also be inhibited by coatings. Wang et al. 47 reported a competition for the contaminant between reactive sites on nanometal and sorption sites on polymer coating. Coatings can also suppress or improve the H 2 generation. Loghmani et al. 81 reported a significant decrease in H 2 generation when Cu-Fe nanosheets were coated with triton X-100 or sodium dodecyl sulfate but the H 2 generation increased with the coatings of polyethylene glycol or polyvinyl pyrrolidone. However, reactivity loss must be weighed against the benefits provided by coating. Although 1,2-DCA removal was lesser and slower for C-nCu 0 W , coating played an important role in keeping particles in suspended form for a longer period. In our study, CMC was chosen as it is a commonly used stabilizer for nanometals during field applications 3,4,27,28 . However, more research is needed to choose the best possible stabilizers which would not only keep particles in suspension but also result in better dechlorination efficiency.
XPS and dechlorination results indicate that washing step is essential for more Cu 0 sites and better catalytic activity of nCu 0 . However, washing step may not be feasible during field-scale synthesis. Other field-applicable changes in synthesis process of nCu 0 need to be studied to improve its catalytic activity. During synthesis, formation of an oxide layer around nanoparticles might be controlled by using an alcohol as a solvent instead of water. Past studies reported successful synthesis of nCu 0 using a range of alcohols (C 1 -C 4 ) as solvents for preparing precursor solutions 39,82 . Alcohols and organic acids (e.g., formic acid, ascorbic acid) are also used as reducing agents to synthesize nCu 0 and can be an attractive alternative to NaBH 4 43,44,49,82 . They also act as successful coatings by providing stability to nanoparticle suspensions. Moreover, organic acids and alcohols can also be used as a H 2 source in catalytic reduction treatments 10,15 .
pH and oxidation-reduction potential (ORP). The hydrolysis of NaBH 4 and consequently the H 2 generation can be significantly influenced by pH 81,83 . As no buffer was used to control the pH in our experiments to avoid any complexity, the initial pH ranged between 9.18 and 9.93 which increased to > 10.5 (10.5-10.8) at the end of the experiments (SI Table S4). These highly alkaline conditions would have been caused by the generation of NaOH from the hydrolysis of water-soluble NaBO 2 (Eq. 17), which comes from NaBH 4 hydrolysis (Eq. 3) 83 . Alkaline conditions precipitate out NaBO 2 , resulting in the blockage of reactive sites which would hinder the H 2 generation 84 and consequently the contaminant removal. The alkaline pH might not have any major impact on the 1,2-DCA removal in our study as a continued decrease in the 1,2-DCA concentrations was observed after re-spiking of NaBH 4 at 24.5 h in experiments 7, 10, and 13 (Fig. 4). With Cu 0 and borohydride, Raut et al. 10 also did not observe any effect on chlorobenzene (> 90%) dechlorination at the highly alkaline pH of 10-12. With copper nanowire as a catalyst at 298 K, Hashimi et al. 83 also did not observe any effect of pH increase from 10.45 to 12 on the H 2 generation but it completely stopped at pH 13.
ORP is the other parameter which can influence the chemical reduction reactions. The initial ORP values for the nCu 0 suspension in our experiments ranged between -752 and -880 mV (Table S4) Dechlorination products/pathways. 85-90% chloride recovery (Fig. 4C) confirmed complete dechlorination of most of the removed 1,2-DCA in bare nCu 0 experiments. The degradation products analysis found that ethane was the major end product, with chloroethane as an intermediate, for all experiments. At the end of experiment 15 (C-nCu 0 ), ethane and chloroethane made up 55.2% and 43.4%, respectively, of the total byproducts along with trace amounts of ethene, acetylene, propane, i-butane, and n-butane, yielding a carbon mass balance (CMB) of ~ 80%. Similarly, ethane and chloroethane contributed 73.8% and 21.7%, respectively, along with ethene (2.5%) and trace amounts of other hydrocarbons, achieving ~ 98% CMB for C-nCu 0 W treatment (Experiment 19; SI: Figure S9). 1,2-DCA was dechlorinated through two successive hydrogenolysis steps (Fig. 7). Direct 1,2-DCA reduction to ethane might also have occurred simultaneously as ethane generation was observed from the beginning of experiment. Shee et al. 14 reported formation of 15, 63, 12, and 3 mol-% of chloroethane, ethane, ethene, and butane, respectively during 1,1-DCA dechlorination via bare nCu 0 -borohydride. Huang et al. 17 also reported ethane as major product with Cl − release while treating 1,2-DCA with bare washed nCu 0 -borohydride. They did not measure chloroethane but suggested its generation to be further investigated. In our study, pres- www.nature.com/scientificreports/ ence of trace amounts of ethene suggests β-elimination as a minor pathway (Fig. 7). However, ethane formation through ethene hydrogenation cannot be ruled out. Previous studies suggested that 1,2-DCA undergoes C-Cl bond dissociation on Cu sites, resulting in the formation of adsorbed •CH 2 -CH 2 • species which readily desorbs as ethene 85 . For C 3 and C 4 byproducts, aqueous-phase studies indicated that carbon-carbon coupling can also occur during hydrodechlorination through a radical mechanism 86 .

Conclusions
Our study provides insights into how synthesis parameters and groundwater solutes influence the characteristics and catalytic dechlorination efficiency of nCu 0 . TEM and SEM/EDX confirmed the formation of nano-sized particles. Metallic (Cu 0 ) nature of particles was confirmed by XRD and XPS. XPS further revealed that washing of nCu 0 particles, after synthesis, increased the number of Cu 0 sites which are responsible for its catalytic activity. The nCu 0 particles were incapable of utilizing H 2 , from residual borohydride from synthesis, for 1,2-DCA dechlorination and rather fresh borohydride injections were essential. The washing step resulted in higher and faster 1,2-DCA dechlorination whereas the CMC coating decreased the dechlorination by nCu 0 -borohydride. Ethane and chloroethane were the main dechlorination products indicating hydrogenolysis as the major pathway. Presence of sulfides, chlorides, and humic acid partially deactivated the nCu 0 particles, resulting in slower and lesser dechlorination. However, additional borohydride injection regenerated the Cu 0 reactive sites and improved the dechlorination efficiency.
Copper is a cheaper alternative to palladium catalyst to successfully degrade the recalcitrant cVOCs via aqueous-phase catalytic reduction. However, it is a toxic element with drinking water limits (DWL) of 2 mg L −187 and, thus, has some limitations for groundwater treatment. Of note, however, is that contaminanted sites of interest typically have contaminants with greater toxicity than copper. More work is required to design the nCu 0 catalysts which result in minimum Cu 2+ leaching to keep the treated groundwater concentrations below DWL. Some lab-scale studies reported successful in situ Cu 0 synthesis and, consequently, complete removal of the co-contaminants 19,40 . There are numerous sites in the world where copper is present as a co-contaminant with other organic and inorganic contaminants. Thus, possibility of generating in situ nCu 0 at the contaminated sites, for catalytic reduction of co-contaminants, can also be explored.

Data availability
The datasets used in the current study are available from the corresponding author on reasonable request.